In diploid cells of Saccharomyces cerevisiae, genetic recombination and recombinational repair are controlled in part by the mating-type locus (mat). Diploids heterozygous (a/alpha) at mat exhibit greater mitotic recombination and resistance to MMS and X-rays than homozygous mat (a/a or alpha/alpha diploids. Closely related a/alpha and alpha/alpha diploids and an abnormal alpha/alpha strain previously isolated will be used to investigate whether mat-mediated MMS repair is inducible and/or associated with DNA synthesis in a/alpha cells. One or more nucleases whose synthesis/activity is influenced by mat will be identified by comparisons of enzymatic activities in fractions from extracts of a/alpha and alpha/alpha diploids. The biological roles of such enzymes will be assessed by determining their activity in cells in different physiological conditions or in mutant cells (the abnormal a/alpha, radiation-sensitive, etc.). We will also investigate the genetic, physiological, and biochemical properties of several types of mutants with altered mat regulation in order to gain more understanding of this complex system and locus. One part of mat control in haploids includes the production of and response to mating hormones. Possible interactions between two or more of these hormones that may function to create an optimal balance of activities in mating mixtures will be studied, as will the physiological properties of cells that have responded to hormone action. Thus, the mating-type system provides an opportunity to investigate at a molecular level question of recombinational repair, hormone action, and their regulation in a model eukaryote.